CentOS SSH Installation And Configuration

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How do I install and configure ssh server and client under CentOS Linux operating systems?

You need to install the following packages (which are installed by default until and unless you removed it or skipped it while installing CentOS)

  • openssh-clients : The OpenSSH client applications
  • openssh-server : The OpenSSH server daemon

OpenSSH Installations under CentOS Linux

To install the server and client type:
# yum -y install openssh-server openssh-clients
Start the service:
# chkconfig sshd on
# service sshd start
Make sure port 22 is opened:
# netstat -tulpn | grep :22

Firewall Settings

Edit /etc/sysconfig/iptables (IPv4 firewall),
# vi /etc/sysconfig/iptables
Add the line
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
If you want to restict access to 192.168.1.0/24, edit it as follows:
-A RH-Firewall-1-INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 22 -j ACCEPT
If your site uses IPv6, and you are editing ip6tables, use the line:
-A RH-Firewall-1-INPUT -m tcp -p tcp --dport 22 -j ACCEPT
Save and close the file. Restart iptables:
# service iptables restart

OpenSSH Server Configuration

Edit /etc/ssh/sshd_config, enter:
# vi /etc/ssh/sshd_config
To disable root logins, edit or add as follows:
PermitRootLogin no
Restrict login to user tom and jerry only over ssh:
AllowUsers tom jerry
Change ssh port i.e. run it on a non-standard port like 1235
Port 1235
Save and close the file. Restart sshd:
# service sshd restart

See also:

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

16 comment

  1. THANKS A MILLION, MAY GOD BLEEEEEEEEEESSSSSSSSSSSSSSSSSSSS YOU.
    YOU JUST SAVED MY LIFE.

    MY HOSTING SERVER’S SSH WAS UNINSTALLED BY MISTAKE AND YOU JUST MADE THE SOLUTION CHEAP

    THANKS AGAIN

  3. There are a couple/few things I don’t get…
    e.g. for

    Make sure port 22 is opened:
    # netstat -tulpn | grep :22

    What are we *supposed* to see if port 22 *is* opened?

    And for

    Add the line
    -A RH-Firewall-1-INPUT -m state –state NEW -m tcp -p tcp –dport 22 -j ACCEPT

    add it *where*???
    In the :INPUT ACCEPT section?
    in the :OUTPUT ACCEPT section?
    after :COMMIT?

    No matter which line I put it on, when I restart the iptables service I get a red [FAILED] message for that line#.

    And then you change the SSH port to 1235 but don’t revisit iptables?

    What am I missing here?

    :-)

  4. Thanks a lot!

    Only change :

    -A RH-Firewall-1-INPUT -s 192.168.1.0/24 -m state –state NEW -p tcp –dport 22 -j ACCEPT

    TO

    -A INPUT -s 192.168.1.0/24 -m state –state NEW -p tcp –dport 22 -j ACCEPT

  6. So,
    Make sure port 22 is opened:
    # netstat -tulpn | grep :22

    What are we *supposed* to see if port 22 *is* open?

    Thanks.

    1. If sshd is running. you would expect to see something like:

      # netstat -plunt | grep :22
      tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 70813/sshd
      tcp 0 0 :::22 :::* LISTEN 70813/sshd
      #

      If you do not get any output where the IP/port part ends in :22, then you have nothing listening on port 22 (the standard ssh port).

      Regards,

  8. Hi,
    Some thing is wrong.
    Please help resolve it.
    The line 13 is this:
    -A RH-Firewall-1-INPUT -m state –state NEW -m tcp -p tcp –dport 22 -j ACCEPT

    Now when is restart iptables i get this error.

    # service iptables restart
    iptables: Flushing firewall rules: [ OK ]
    iptables: Setting chains to policy ACCEPT: filter [ OK ]
    iptables: Unloading modules: [ OK ]
    iptables: Applying firewall rules: iptables-restore: line 13 failed [FAILED]

  10. You missed something important: If you alter SSH Port, your iptables line –dport needs to be the same as the port you specified, not 22!

    So, if you set: Port 234, then your IP tables entry would be this:
    -A INPUT -m state –state NEW -m tcp -p tcp –dport 234 -j ACCEPT

    To verify your PROPER port is open, the command also needs to reflect the port you altered: netstat -tulpn | grep :234

    Your results should look like this:
    tcp 0 0 0.0.0.0:234 0.0.0.0:* LISTEN 26873/sshd
    tcp 0 0 :::234 :::* LISTEN 26873/sshd

    The author should not have suggested changing the port without explaining that the commands and IPTables entries would necessitate being altered as well.

    1. I should have read this comment about 30 minutes ago… lol I was however, able to figure it out on my own. I guess that’s a good thing. :o)

  13. Any changes to iptables *** must *** be on lines above COMMIT, which is found at the end of the file.
    Before adding the port (eg 22), do a search to see if it already exists:
    nano iptables
    Ctrl W to invoke search
    type: dport 22

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